This is a very interesting question. I don't even know whether it actually is good for children to meditate. "Inner dialogue" may be necessary for a young mind to develop. I believe development of mindfulness and silencing of the inner dialogue is a stage of development a person is meant to reach as they become adults. Those who don't reach it become old while keeping immature.

Correct me if mistaken but isn’t the goal of mindfulness meditation to accept the inner dialogue rather than fighting it?

For instance at one point as a child I had a conscience that would talk to me but as I grew older the voice passed into the background. Now my thoughts are the sort of jumbled mess meditators talk about.

So I don’t understand why developing the ability to hear and accept these other voices would be a detriment.

It's not the goal, it's the way. It disappears as you actually learn to observe it and everything mindfully all the time. E.g. now I only have inner dialogue when I choose to entertain myself this way and some times I feel too lazy even for this.